The present invention relates to a method for the purification of polluted water, such as waste water, by the activated sludge method, whereby the polluted water is successively subjected to biological treatments in at least two treatment zones, and whereby the water thus treated is passed to a permanent clarification zone and separated into a water fraction and a sludge fraction, at least a portion of the latter being recycled and mixed with untreated water.
A known method of the above-mentioned type and which is described in GB 1 404 565, is used for the removal of nitrogen from waste water. In this known method, two biological treatment zones are used, and a mixture of untreated waste water and recycled sludge is in one period passed to the one treatment zone during which period stirring is carried out while simultaneously maintaining anoxic conditions, and subsequently to the second treatment zone where, during the same period, aeration is carried out while maintaining aerobic conditions.
During a subsequent period the mixture of waste water and recycled sludge is passed to the second treatment zone where anoxic conditions are now maintained, and then to the first treatment zone where aerobic conditions are maintained during the same period.
During the aerobic treatment of the waste water in the presence of the micro-organisms contained in the sludge, NH.sub.3 -nitrogen is converted to nitrate (nitrification). When the nitrate thus formed is subsequently subjected to anoxic conditions in the presence of micro-organisms, nitrate is converted into free nitrogen (denitrification), the nitrogen thus formed being removed in gaseous form.
According to DK-B-149 767, the above-described known method can be combined with an initial anaerobic treatment of the mixture of untreated waste water and recycled sludge. By this known method, both phosphorus and nitrogen may be removed from the waste water.
In a further known method of the kind described in the introductory part--the so-called Tricycle-method--a mixture of untreated waste water and recycled sludge is, following initial anaerobic treatment, subjected to successive treatments in three different treatment zones before the waste water thus treated is passed to the clarification zone.
In a first cycle, the anaerobically treated mixture of waste water and sludge is subjected to an anoxic treatment in the first of the three treatment zones and then to successive aerobic treatments in the second and the third treatment zones.
In a subsequent cycle, the anaerobically treated mixture of waste water and sludge is passed to the third treatment zone and is therein subjected to an anoxic treatment before it is passed to the first and subsequently to the second treatment zone in which it is subjected to successive aerobic treatments.
In a third cycle, the anaerobically treated mixture of waste water and sludge is passed to the second treatment zone in which it is treated anoxically and then to the third and finally to the first treatment zone in which aerobic conditions are maintained.
In a further known method--the so-called Triple-channel method--three treatment zones are used, and in a first phase the waste water is treated successively in the three zones of which the third act as a clarification zone. In a subsequent phase the untreated waste water is supplied to the third treatment zone and passed through the second treatment zone to the first zone which acts as a clarification zone in this fase. In the known method no permanent clarification zone is used and the precipitated sludge is not recycled from the clarification zone.
It is well known that the efficiency of biological treatments of waste water in the presence of activated sludge increases with increasing amounts of sludge in the treatment zones. However, in order to obtain high sludge concentrations and thus large amounts of sludge in the treatment zones, the mass balance requires operation with high sludge-recycling ratios, i.e. high ratios between the amount of sludge recycled from the clarification zone and the amount of untreated waste water.
The relationship between sludge concentration in the treatment zones and the recycling ratios will appear from Table 1, which is based on the assumption that the sludge concentration in the return sludge is about 8 kg/m.sup.3.
TABLE 1 ______________________________________ Sludge Concentration kg/m.sup.3 Recycling Ratio ______________________________________ 1 0.15 2 0.34 2.5 0.50 3 0.60 3.5 0.75 4 1.0 4.5 1.25 5 1.50 5.5 2.10 ______________________________________
As will appear from Table 1, it is necessary to use a sludge recycling ratio of 1.0 in order to obtain a sludge concentration of about 4 kg/m.sup.3. This involves partly a high load on the clarification zone and partly substantial costs for the construction and operation of the sludge recycling equipment.
The invention is based on the discovery that high sludge concentrations may be obtained in the biological treatment zones without using high recycling ratios and while avoiding the problems associated therewith by maintaining for a period such conditions in one of the treatment zones that a sludge preciptiation occurs therein and by supplying, in a subsequent period, the mixture of polluted water and recycled sludge directly or indirectly to the zone which, during the preceding period, served as a sludge precipitation zone prior to further treatment thereof in one or more treatment zones.
By effecting the water purification in the manner described above, comparatively large amounts of sludge will be concentrated in one of the treatment zones, and instead of passing this sludge to the permanent clarification zone which an actual separation of sludge is effected and from which the separated sludge is recycled and admixed with untreated water, this treatment zone having a high sludge content is used as a biological treatment zone for incoming polluted water in a subsequent period. Hereby a highly increased ratio of sludge to polluted water and thus an increased purification effect is obtained. In other words, sludge is withheld and the withheld sludge is "moved" to the upstream side of the witholding site rather than being recycled in a conventional manner.